A carbonaceous material has been normally used as a negative electrode active material of nonaqueous secondary batteries. However, a carbonaceous material has difficulty on account of its low theoretical discharge capacity in dealing with the increase of power consumption of small-sized electric/electronic equipment and the use in an on-board battery.
Silicon-containing materials having a higher capacity than carbonaceous materials have been studied as a negative electrode active material taking the place of the carbonaceous materials. For example, patent literature 1 below proposes using a boron-containing silicon material as a negative electrode active material. According to the disclosure, mixing a silicon material powder having a suitable particle size with a low concentration of boron and heating the mixture under a certain condition provides a boron-containing silicon material powder having a considerably smaller amount of silicon boride (SiB4) than thermodynamically estimated from the boron content and assuming a substantially supercooled state. Using such a boron-containing silicon material powder is described as providing markedly reduced capacity loss and improved cycle characteristics while retaining the large discharge capacity inherently possessed by silicon.
Another negative electrode active material containing silicon and boron is known from patent literature 2 below. According to the disclosure, the intended active material is obtained by introducing a molten alloy containing silicon and boron into a swirling water flow either directly or after being atomized by gas into droplets thereby to rapidly cool the molten alloy. FIG. 3 of patent literature 2 shows a cooling velocity in the order of 105K/second.